1. Technical Field
This invention relates to a process for producing a zinc-plated steel sheet with an ageing resistance by hot dip-type, continuous zinc plating of an Al-killed steel, and more particularly to a process for producing a continuous zinc-plated steel sheet with an ageing resistance from an Al-killed steel by use of a special heat cycle in the course of quenching after the recrystallization and annealing and overageing after the dipping in a zinc plating bath.
2. Description of the Prior Art
Conventional processes for producing a zinc-plated steel sheet with an ageing resistance by hot dip-type, continuous zinc plating include a process by hot dip type, continuous zinc-plating of IF steel (Interstitial Free Steel) as a starting material and a process based on hot dip-type, continuous zinc-plating of Al-killed steel as a starting material, followed by post box annealing.
The former process by hot dip-type, continuous zinc-plating of IF steel as a starting material uses IF steel, that is, an extremely low carbon steel containing Ti, Nb, etc. as alloy elements, for the non-ageing purpose and has such a disadvantage as a high production cost for making the carbon content extremely low and adding alloy elements such as Ti, Nb, etc. to the steel. On the other hand, the latter process using an Al-killed steel as a starting material requires the post box annealing after the step of hot dip-type, continuous zinc plating, that is, one more step is required, resulting in such a serious drawback as an excessive production cost. Thus, it has been keenly desired to develop a process for producing a zinc-plated steel sheet with an ageing
Incidentally, several attempts have been so far made regarding a process for producing a cold rolled steel sheet with a good ageing resistance by continuous annealing from an Al-killed steel as a starting material, in which the Al-killed steel is not further subjected to hot dip-type, continuous zinc plating. For example, it has been proposed to improve the ageing resistance by devising a heat cycle for operations from quenching down to overageing by the following prior arts: Japanese Patent Publication No. 58-10447 disclosing a horizontal overageing process, in which the steel sheet reheated after the supercooling is retained at the same temperature in an operation after the reheating and a relationship between the temperature and time in such an operation of retaining for overageing is composed of a horizontal linear line; and the Japanese Patent Publication No. 58-39890 and Japanese Patent Application Kokai (Laid-Open) Nos. 60-52527 and 61-276935 disclosing an overageing process according to an inclinatory cooling, in which the temperature of the steel sheet reheated after the super-cooling is changed with the passage of time in an operation after the reheating or the subsequent retaining and a relationship between the temperature and time in such an operation of cooling for overageing is expressed by an inclinatory line. However, the proposed processes, in which the steel sheet is not further subjected to hot dip-type, continuous plating, also fail to produce a cold rolled steel sheet having a good ageing resistance from an Al-killed steel, because the heat cycle concept as well as the heat cycle itself has defects.